Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/46—Removing components of defined structure
  • B01D53/48—Sulfur compounds
  • B01D53/50—Sulfur oxides
  • B01D53/501—Sulfur oxides by treating the gases with a solution or a suspension of an alkali or earth-alkali or ammonium compound
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
  • C01F11/00—Compounds of calcium, strontium, or barium
  • C01F11/46—Sulfates
  • C01F11/464—Sulfates of Ca from gases containing sulfur oxides

Definitions

• This invention relates to a process for preventing the formation of gypsum scale in a flue gas desulfurization process.
• an exhaust gas is brought into contact with an absorbing solution containing Ca(OH) 2 in a scrubber whereby SO 2 contained in the exhaust gas reacts with Ca(OH) 2 and is absorbed. According to this reaction, there may be obtained CaSO 3 ⁇ 1/2H 2 O, part of which in turn is oxidized so as to give gypsum.
• the concentration of Ca(OH) 2 to be used may be estimated according to the following equation: ##EQU1## wherein G: amount of exhaust gas (Nm 3 /hr)
• L/G represents a liquid-to-gas ratio (1/Nm 3 ), which is generally governed by the performance of the scrubber.
• U.S. Pat. No. 3,959,441 filed by the inventors discloses a flue gas desulfurization process using Ca(OH) 2 , in which CaCl 2 is used for increasing the solubility of Ca(OH) 2 and for clearing the absorbing solution.
• the concentration of Ca(OH) 2 in this case ranges from 0.3 to 0.8% by weight.
• gypsum scale is formed when gypsum produced in the scrubber crystallizes in the form of a solid from a liquid, and then clings to the inner surfaces of the scrubber and piping in the form of crystal nuclei, and then gypsum crystallizes subsequently on the crystal nuclei thus formed, so that gypsum crystals grow and form extremely hard scale.
• a process for preventing the formation of gypsum scale in which, upon the desulfurization treatment of gas containing SO 2 according to the lime-gypsum process, with the concentration of Ca(OH) 2 in the absorbing solution being not more than 0.1% by weight, CaCl 2 is contained in the absorbing solution in amounts of not more than 40% by weight.
• the concentration of Ca(OH) 2 in the absorbing solution ranges from 0.001 to 0.1% by weight.
• a process for preventing the formation of gypsum scale according to the first aspect of the invention in which CaCl 2 is contained in the absorbing solution in amounts ranging from 1 to 40% by weight, preferably from 5 to 35% by weight.
• the concentration of Ca(OH) 2 contained in the absorbing solution is not more than 0.1% by weight.
• concentration of Ca(OH) 2 exceeds 0.1%, then unreacted Ca(OH) 2 remains in the absorbing solution which is discharged from the scrubber in a conventional flue gas desulfurization plant, so that there is produced little or no gypsum scale. Accordingly, the above case in outside the technical scope of the present invention.
• an absorbing solution is used within the aforesaid range, the prevention of formation of gypsum scale by sprinkling of liquid or stability of the pH value dictates the use of an absorbing solution in great amount, thus leading to lack of economy.
• the addition of CaCl 2 is mandatory.
• the concentration of Ca(OH) 2 ranges from 0.001 to 0.1% by weight.
• the concentration of Ca(OH) 2 may be less than 0.001% by weight in case the concentration of SO 2 is relatively low and the performance of the scrubber is satisfactory.
• the concentration of Ca(OH) 2 may be over 0.1% by weight with some success, in case gypsum is produced in the absorbing solution.
• the amount of CaCl 2 as used in the process of the invention should be no more than 40% by weight, preferably from 1 to 40%, and more preferably from 5 to 35% by weight.
• two aqueous solutions containing CaCl 2 and Ca(OH) 2 may be prepared separately, and then mixed with each other.
• Ca(OH) 2 in the form of powder may be supplied to a CaCl 2 aqueous solution at a time, thereby providing an aqueous solution containing CaCl 2 and Ca(OH) 2 .
• the contents of CaCl 2 and Ca(OH) 2 are suitably selected within the aforesaid range of concentration, commensurate with the concentration of SO 2 , liquid-to-gas ratio and other operational conditions.
• the absorbing solution thus prepared is fed into a scrubber, so as to contact an exhaust gas by means of a gas-liquid-contacting mechanism in the scrubber, for instance spray apparatus or the like, so that SO 2 gas in the exhaust gas may be absorbed into the absorbing solution which in turn is delivered to the treatment section such as for a gypsum forming treatment.
• the solubility of CaSO 4 formed in an absorbing solution is lowered with the addition of CaCl 2 .
• a solubility of CaSO 4 is lowered to about 1/100 of the solubility of CaSO 4 in water, at a concentration of CaCl 2 of 30% by weight. This causes CaSO 4 to crystallize in the liquid quickly, and decreases the amount of CaSO 4 which is present in the super-saturated condition.
• gypsum crystallizes in a liquid so as to form new crystals
• gypsum crystallizes on crystals which have been formed in a liquid, thereby allowing the growth of crystals
• gypsum is dissolved in a liquid in a super-saturated state and then crystallizes on crystal nuclei on the inner surfaces of piping.
• the former two cases wherein gypsum is newly formed provide no possibility of the formation of hard scale.
• What is the problem is the last case wherein gypsum crystallizes on crystal nuclei. Accordingly, a decrease in amount of CaSO 4 present in the super-saturated state directly leads to the prevention of formation of hard scale.
• a gas which contained about 300 ppm of SO 2 and about 15% O 2 and was discharged from a sintering plant furnace at a temperature of 110° to 120° C. was introduced into a scrubber at a rate of 350,000 Nm 3 /HR, while an absorbing solution containing about 30% by weight of CaCl 2 , and 0.02% by weight of Ca(OH) 2 was introduced into the scrubber at a liquid-to-gas ratio (L/G) of 3 1/Nm 3 for the absorption of SO 2 .
• L/G liquid-to-gas ratio
• the flue gas desulfurization plant was operated for eight months under the above conditions.
• the initial SO 2 -removal efficiency was about 90%, and this value was maintained even after eight months, while the pressure drop of gas in the scrubber was maintained at 80 to 100 mm of water.
• Table 1 shows the results of a desulfurization treatment for an exhaust gas coming out of a sintering plant furnace, in which an absorbing solution according to the present invention is used in the same manner as in the preceding example.
• a flue gas desulfurization plant was operated for one month using an absorbing solution comprising a Ca(OH) 2 aqueous solution containing no CaCl 2 .
• the initial SO 2 removal efficiency was about 90%, no different from the case of the absorbing solution containing no CaCl 2 .
• the SO 2 removal efficiency was lowered to about 80%, while the pressure drop was increased to 200 mm of water.
• the concentration of Ca(OH) 2 is set to a level equivalent to or less than that of the SO 2 contained in the exhaust gas, thereby preventing the discharge of unreacted Ca(OH) 2 .
• the addition of CaCl 2 reduces the solubility of CaSO 4 , thereby causing CaSO 4 to crystallize immediately in the liquid, while lowering the degree of super-saturation thereof, in order to prevent the formation of CaSO 4 scale on the side walls of the scrubber or in piping.
• troubles such as a lowered absorption efficiency of SO 2 gas within the scrubber and clogging of piping may be solved in a simple manner economically.
• the absorbing solution due to the addition of CaCl 2 thereto, has a lower vapor pressure and a higher boiling point, with the result that the temperature of the exhaust gas, when adiabatically cooled, is increased, thereby reducing fuel consumption for after-burning.
• gas after desulfurization is re-heated to 100° to 140° C. before being discharged into the atmosphere, for the purposes of protecting materials used for stacks and preventing a white smoke.
• the cost of fuel required for the above reheating amounts to about 30% of the operating cost of the apparatus.
• the achievement of a high temperature clean gas according to the present invention greatly contributes to the reduction of the operating cost.
• the process according to the present invention provides various advantages in flue gas desulfurization by the lime-gypsum process.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Organic Chemistry (AREA)
• General Chemical & Material Sciences (AREA)
• Environmental & Geological Engineering (AREA)
• Analytical Chemistry (AREA)
• Health & Medical Sciences (AREA)
• Oil, Petroleum & Natural Gas (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Life Sciences & Earth Sciences (AREA)
• Geology (AREA)
• Inorganic Chemistry (AREA)
• Biomedical Technology (AREA)
• Treating Waste Gases (AREA)

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Cited By (7)

Citations (4)

• 1976
  • 1976-12-21 DE DE19762657971 patent/DE2657971A1/de active Granted
• 1977
  • 1977-09-08 BE BE180768A patent/BE858519R/xx not_active IP Right Cessation
  • 1977-12-19 US US05/861,973 patent/US4213946A/en not_active Expired - Lifetime

Patent Citations (4)

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